The invention relates to a chair, particularly an office chair of the type having a seat plate that can be adjusted in its slope and a backrest which is adjustable as a function of the seat plate slope adjustment.
From German Application No. (DE-OS) 33 16 530, a chair is known, the backrest of which carries out an adjusting motion as a function of the slope of the seat plate via a control linkage. In the case of this chair, when the slope of the seat plate is adjusted, a lifting of the front edge of the seat plate is caused via control rods of this gear. In a comfort-reducing way, this results in a forced change of the original foot supporting point because of a so-called pulling-up of the legs. By means of the backrest that can be swivelled relative to the seat plate, a constant changing of the essential body supporting points at the chair is caused which also results in an effect that is disadvantageous with respect to the correct fitting of the clothes. Also, from DE-OS 34 15 555, a chair has become known that comprises a seat surface made of two parts that are flexibly connected with one another. The part that is located in the front is connected firmly with the seat carrying means and the pivoted rear part of the seat plate is articulated to a two-part backrest, the lower backrest part of which, in the area of the pelvis, is pivotably held at the rear plate part. As a result of this chair development, when the slope of the seat is adjusted, via a control linkage, a relative movement becomes possible between the rear part of the seat surface and the connecting lower seat backrest part, which also, in a comfort-reducing way, causes a friction effect between the backrest of the seat and the user's back.
An objective of the invention consists of providing a chair that, in its basic position and in its inclined positions, ensures a comfortable sitting without changing the user's relevant support points with respect to the chair as well as the leg supporting points at the contact surface of the chair.
According to the invention, this objective is achieved by constructing the chair seat plate with a front plate part and a rear plate part flexibly connected with one another and being supported by scissor shaped seat carrying supports and control coupling elements which accommodate relative pivotal adjustment of the plate parts with pivotal movement of the front plate part about a substantially fixed imaginary axis located in front of the chair at the location of the knees of a person sitting in the chair. In preferred embodiments, this imaginary axis is located at an intersection of a horizontal plane of the seat surface when in a basic horizontal seat adjusted position and an inclined plane of one of the coupling elements when in an inclined front seat part adjusted position.
Principal advantages that are achieved by means of the invention arise from the fact that, by means of the design of the kinematics, the front edge of the seat is lowered when the slope of the chair is adjusted and the seat plate swivels around an imagined axis of rotation located in front of it. This axis of rotation is preferably located in the ara of the user's knees, resulting in an unchanged supporting point for the feet, and at the same time relieving the thighs from load. As a result, according to the invention, an uncomfortable pressing in the hollows of the knees by a lifting of the front edge of the seat according to the state of the art is avoided.
Additional advantages are that the user's relevant support points with respect to the chair, because of the one-piece construction of the rear seat plate with the backrest according to preferred embodiments, are also unchanged when the slope is adjusted and a support of the pelvis is achieved without friction effects, i.e., relative movements between the back and the chair.
By investigating seats, it was found that an ergonomically comfortable holding of the seat is ensured when the front plate part of the seat surface is included to the horizontal with respect to the rear plate part at a ratio of 1:2. When the seat is adjusted by means of the control linkage, this ratio can also be maintained in the intermediate positions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.